1. Field of the Invention
The present invention relates to a path protection switching device in a transmitter-receiver for transmitting and receiving synchronous byte multiplex signals, and more particularly, to a path protection switching device for a transmitter-receiver employing PPSR (Path Protection Switched Ring) of SONET (Synchronous Optical NETwork) type.
2. Description of the Related Art
PPSR is a method for selecting a path (channel) of excellent state from current and reserve lines of a ring network to ensure line quality, and is presently used as a basic technique for new synchronous network transmission. As a signal to be transmitted, a multiplex signal obtained by subjecting a basic signal (STS-12, STS-3, STS-1, VT1.5, VT2, VT3, VT6, etc.) to time-division multiplexing is used. According to PPSR, the current or reserve line is selected for each of the basic signals from which the multiplex signal is derived.
FIG. 13 illustrates a SONET type ring network. As illustrated, ADM (Add Drop Multiplexer) transmitters-receivers 101 to 104 are connected in a ring via a two-way optical transmission path, and synchronous byte multiplex signals are transferred among the transmitters-receivers. The transmitters-receivers 101 to 104 have the same internal arrangement; therefore, the transmitter-receiver 101 alone will be explained below.
The transmitter-receiver 101 comprises line terminators (LTE) 101a and 101b, a path switch 101c, a gate section 101d, and other elements. The line terminator 101a extracts only those signals (drop signals) which are directed to low-order transmit-receive units (not shown) belonging to the transmitter-receiver 101, from among the multiplex signals received from the transmitter-receiver 102, and supplies the extracted signals to the path switch 101c, while the remaining signals (through signals) are transferred to the line terminator 101b. The line terminator 101b derives a multiplex signal by adding signals (add signals) supplied thereto from the low-order transmit-receive units via the gate section 101d, to the through signals supplied from the line terminator 101a, and transmits the multiplex signal to the transmitter-receiver 104. A process similar to the above is executed by the line terminators 101b and 101a with respect to multiplex signals transmitted from the transmitter-receiver 104.
The path switch 101c serves principally to select a signal of higher line quality from the drop signals supplied from the line terminators 101a and 101b, and to supply the selected signal to the low-order transmit-receive units. These drop signals are identical to each other but are transmitted to the transmitter-receiver 101 via different routes.
The internal arrangement of the transmitter-receiver will be now explained.
FIG. 14 is a block diagram showing the internal arrangement of a conventional transmitter-receiver utilizing PPSR. The figure illustrates a drop signal processing section alone.
O/E modules 105 and 106 each convert an optical signal supplied thereto to an electrical signal, and DMUXs 107 and 108 each separate the multiplex signal into n-channel signals. TSA (Time Slot Assignment) elements 109 and 110 individually perform cross connection. The TSA 109 distributes drop signals of the individual channels to n path switches 111-112, as well as to n alarm detecting sections 113-114. Similarly, the TSA 110 distributes drop signals of the individual channels to the n path switches 111-112, as well as to n alarm detecting sections 115-116.
The alarm detecting sections 113-114 each extract alarm information contained in the drop signal supplied thereto, and supplies the extracted information to the corresponding one of the path switches 111-112 as a control signal. Similarly, the alarm detecting sections 115-116 each extract alarm information contained in the drop signal supplied thereto, and supplies the extracted information to the corresponding one of the path switches 111-112 as a control signal. Based on the alarm control signals, each of the path switches 111-112 selects a drop signal of higher line quality from the two drop signals supplied thereto, and supplies the selected signal to the low-order transmit-receive unit of the corresponding channel. For example, the path switch 111 associated with channel 1 selects a drop signal of higher line quality from the two drop signals supplied from the TSAs 109 and 110, based on the control signals supplied from the alarm detecting sections 113 and 115, and supplies the selected signal to the low-order transmit-receive unit associated with channel 1.
The conventional transmitter-receiver described above requires n path switches 111-112, n alarm detecting sections 113-114, and n alarm detecting sections 115-116, corresponding in number to the n channels of the low-order transmit-receive units.
Thus, in the conventional transmitter-receiver, if the number of channels of the low-order transmit-receive units is increased, the scale of circuitry of the transmitter-receiver inevitably enlarges with increase in the number of channels, entailing corresponding increase in the cost of products, power consumption, capacity of the devices and the like.
Further, in cases where extra data need be added to the contents of alarm detection due to a change of the PPSR specifications, for example, the scale of the circuitry must be enlarged corresponding to the number of the channels on each such occasion.